Sealing devices utilizing ferrofluid are known as described, for example, in Japanese Patent Publication No. 24585/1986. Such a sealing device has an annular sealing member to be disposed so as to surround the circumference of a shaft. This sealing member comprises an annular magnet and a pair of pole pieces bonded onto both sides of the annular magnet, with a ferrofluid being interposed between the internal circumferential surfaces of the annular pole pieces and the external circumferential surface of the shaft, whereby to function as a sealer.
In recent years, it has been attempted in such a sealing device to form an electric circuit utilizing an electrically conductive ferrofluid as the ferrofluid, in which electric current flows from the annular pole pieces through the ferrofluid to the shaft, which is, for example, utilized as a grounding circuit for preventing computer hard disks from being charged with static electricity.
When it is desired that such ferrofluid be used as a part of an electric circuit, said part must have the smallest possible resistance. The resistance value in said part depends on the conducting area and the distance current flows in the ferro-fluid. The latter conducting distance: corresponds to the distance between the internal circumferential surface of the annular pole pieces and the external circumferential surface of the shaft, namely to the seal gap, and it is extremely difficult to further reduce the seal gap in view of its relationship with assembling accuracy and the like. On the other hand, the former conducting area is predominantly defined by the area of the internal circumferential surface of the annular pole pieces. Accordingly, in order to increase the conducting area, the thickness of the annular pole pieces must be increased, and thus more compact sealing devices which are in demand cannot be obtained.
This invention has been accomplished for solving the above problems of the prior art and is directed to provide a sealing member for a ferrofluid seal which can reduce the electrical resistance at the section of ferrofluid and form a compact sealing device.
These and other objects of this invention will become more apparent from the description of the following embodiment.